text: When there is no chemical reaction, or considering the unburnt  side of the flame, the mass fraction of fuel and oxidizer are  and  (the subscript  denotes unburnt mixture). This allows to define a local fuel-air equivalence ratio 
  The local equivalence ratio is an important quantity for partially premixed combustion. The relation between local equivalence ratio and mixture fraction is given by
  The stoichiometric mixture fraction  defined earlier is the location where the local equivalence ratio .